The present invention relates to a butterfly valve for media under high pressure and/or at high temperature, consisting of a housing with a passage for the medium and of a butterfly arranged as a shut-off member within the passage and, via a shaft guided at both ends in pivot bearings on the housing, extending transversely through the passage and guided on one side by an actuating end out of the housing to the exterior, pivotable about an axis of rotation in such a way that, in an opened position, its disk plane extends approximately in the direction of the passage and in a closed position, lying transversely to the passage direction, it interacts with a sealing seat on the housing to form a circumferential seal, an advance bearing seal being arranged in front of each of the two pivot bearings on its inner side facing the passage in such a way that an annular gap surrounding the shaft in the region of the respective pivot bearing is sealed off against penetration of medium from the passage.
Such valves are used as shut-off members in high-pressure systems, for example between pipe work sections, pressure vessels, boilers, turbines and/or the like. Customarily, in such cases, an extensive sealing zone of the butterfly and the circumferential sealing seat on the housing are of xe2x80x9cconicalxe2x80x9d design, by way of adaptation to the closing and opening movements of the butterfly, so that, in the closed position, the butterfly is pressed firmly into the sealing seat in the event of the action of pressure from the side on which the shaft extends. As a result, an effective seal is guaranteed even under very high pressure, and at the same time the butterfly is also mechanically supported against the pressure. In practice, however, such valves are generally designed for both directions of flow, so that the reverse case of loading also often arises, the butterfly being subjected to the action of a counter-pressure on its side facing away from the shaft.
In earlier known valves, it has proven in practical use that functional problems may arise with certain media in that, after a certain time, the butterfly becomes almost or even completely impossible to actuate.
In a butterfly valve of the generic type described in DE-U-298 22 791, therefore, provision is made for each of the two pivot bearings on the housing to have a statically compressed advance bearing seal between the shaft and the housing in its inner end region facing the passage. The intention here is to prevent the respective pressurized medium from penetrating from the passage into the region of the pivot bearings, in other words, in each case, into an annular gap between the shaft and bearing hole in the housing. This measure is based on the finding that it is precisely this kind of penetration by certain media into the region of the pivot bearings that causes the problems described. In particular, polymerizing media (known as xe2x80x9ccrack mediaxe2x80x9d), such as hydrocarbon and certain resins, may cause deposits, especially in the form of crystals, and so result in seizing of the shaft in the pivot bearings. The intention is to prevent this by means of the advance bearing seal, the media not being able to penetrate into the region of the pivot bearings in the first place. However, it has been found that such problems may still occur after extended periods of use.
It is an object of the present invention further to improve a butterfly valve of the generic type so that, even after extended periods of use, and use for any desired media, problem-free functioning for the opening and closure of the passage can always be guaranteed.
This is achieved, according to the invention, in that (at least) the advance bearing seal lying remote from the actuating end of the shaft can be axially subjected via tensioning means to the effect of a retensioning force such that a subsequent radial compressibility is achieved. Preferably, the other advance bearing seal lying closer to the shaft actuating end can also be acted upon via tensioning means for retensioning. It is particularly advantageous here for the tensioning means to be formed by spring means, in such a way that the, or each, advance bearing seal is permanently subjected to the action of spring force in the axial direction, so that a continuous self-adjusting radial compression is achieved.
The invention is based on the finding that, in the known, generic butterfly valve, the material qualities of the graphite gaskets and/or (carbon) fiber meshings used in the region of the advance bearing seals, after extended use under high pressure and/or at high temperature, may be affected by settling phenomena such that a static sealing compression existing there, previously predetermined by means of screw connections may slacken. It is admittedly possible in principle, in the known butterfly valve, for the advance bearing seal lying closer to the actuating end of the shaft to be retensioned via screw connections. However, this not only requires maintenance work, though work whose necessity cannot be perceived at all from outside, but also the other advance bearing seal is tensioned exclusively statically, via a housing lid, and therefore cannot be retensioned at all, even by maintenance. This can result in a lack of tightness of both advance bearing seals, though particularly of the advance bearing seal lying remote from the actuating end of the shaft, as a result of which the medium concerned can again penetrate into the area of the pivot bearings and cause the problems described initially.
By contrast, what is achieved by the invention is that both advance bearing seals can now be retensioned, at least by maintenance work. In the preferred embodiment, in which spring means are provided as tensioning means, automatic self-adjustment of the sealing compression is advantageously achieved in that the spring means subject the advance bearing seals to virtually constant spring force at all times and so brace them permanently. As a result, a virtually maintenance-free butterfly valve with a long service life is provided. In addition, the further advantage arises that, because the advance bearing seal is maintained over a long period, the axial application of force to the shaft, caused by pressure is avoided, because the pressure medium is kept away from a space in which the end of the shaft opposite an actuating end is virtually encapsulated; this space is advantageously kept free of pressure by the effective, self-adjusting advance bearing seal.
In a preferred embodiment of the invention, it is envisaged that each advance bearing seal has a two-part bearing bush with an inner bush directly receiving the shaft and an outer bush receiving the inner bush and seated in a bearing hole in the housing. In this case, a compressible sealing element is arranged in the inner end region, in other words the end region facing the passage, of the respective pivot bearing, axially between the inner bush and a projecting end section of the outer bush in the form of an annular web, so that, as a result of the axial compression of the bearing bushxe2x80x94or of the inner bush and the outer bush relative to one anotherxe2x80x94the sealing element is also radially compressed and continuously inwardly compressed against the shaft by the spring force according to the invention. This preferred embodiment represents a particular advantage in terms of the assembly of the butterfly valve. The shaft can first be easily mounted by simply being pushed into the pivot bearingsxe2x80x94preferably formed by the respective two-part bearing bushes. Only after this assembly are the advance bearing seals then braced by axial compression of the bearing bushes by the spring means.
Further advantageous design features of the invention are contained below in the description that follows.